1. Field of the Invention
The invention relates to a class of novel Brønsted acidic ionic liquids each having a N-protonated lactam cation and to a method for preparing the same.
2. Description of the Related Art
Room temperature ionic liquids (RTILs) are molten salts with a melting point at or below 100° C., which are generally composed of an asymmetric organic cation, and an inorganic or organic anion. (P. Wasserscheid, T. Welton, Ionic liquids in Synthesis, Wiley-VCH, 2003). In comparison with solid materials, RTILs are liquid; in comparison with conventional liquid materials, RTILs are completely composed of ions. Therefore, RTILs always exhibit specific physicochemical properties and functions that conventional solid or liquid materials are not possessed of. RTILs possess many characteristics, such as nonvolatile or ‘zero’ vapour pressure, for which RTILs are regarded as green solvents; low melting point (can be lower than about 90° C.); wide liquid range (can be wider than 200° C. or above) and strong electrostatic field, which is a typical characteristics distinguished from molecular media and materials; wide electrochemical window (can be wider than 5V), which means that RTILs show no electrochemical reactions within such wide voltage range, i.e. degradation; good ionic conductivity (up to 2.5×10−2 S/cm) and thermal conductivity; high heat capacity and heat storage density (6.4 times higher than that of heat storage oil used at present, the heat storage density of the heat storage oil is 9.4 MJ/m3); high thermal stability (decomposition temperature can be up to 400° C.); selective dissolubility, which made RTILs be called “liquid zeolite”, and designability. These specific characteristics make RTILs could be “solid liquid” accompanied with characteristics of liquids and solids simultaneously. Theoretically, there are more than several million billion ionic liquids which could be prepared. The combinations of the ionic liquids diversity and their varied functions and characteristics make it possible to synthesize a great deal of functional media and materials having different properties and applications.
In 1948, U.S. Pat. Nos. 4,446,331, 4,446,349, and 4,446,350 disclosed ethylpyridinium halide-aluminium trichloride room temperature ionic liquids which could be used in plating. This can be regarded as the first generation of ionic liquids. However, these RTILs are moisture and air sensitive and are liable to decompose when mixed with water. In 1990s, more stable RTILs each composed of a dialkylimidazolium cation and a anion of BF4− or PF6− were successfully prepared, which emblematized the naissance of the second generation of ionic liquids (Wilkes J S et al, Air and Water Stable 1-ethyl-3-methylimidazolium Based Ionic Liquids. J. Chem. Soc., Chem. Commun., 1992, 965–967). Due to suitable chemical structure and ideal physicochemical properties of dialkylimidazolium cation, these ionic liquids based on imidazolium have been studied extensively for many years up to the present.
The types and functions of ionic liquids based on imidazolium have been further enriched since 2000 by introducing functional group to the side chain of cations, and a series of task-specific ionic liquids have been prepared, which have endowed these ionic liquids with special properties, functions and applications (A. Wierzbicki, et al, Proceedings of the Symposium on Advances in Solvent Selection and Substitution for Extraction, Atlanta, Ga., Mar. 5–9, 2000). For example, dialkylimidazolium ionic liquids containing an amino group at the end of side chain can trap carbon dioxide at room temperature and release it under high temperature, which could be applied for selective separation of carbon dioxide from gases mixture (Eleanor D. Bates et al, J. Am. Chem. Soc., 2002, 124, 927); dialkylimidazolium ionic liquids each having an urea or thiourea group can coordinate Cd2+ and Hg2+ selectively to achieve extraction and separation from the solutions (Ann E. Visser et al, Environ. Sci. Technol., 2002, 36, 2523–2529); dialkylimidazolium ionic liquids each having a strong Brønsted acidic group such as —SO3H have been used as catalyst and/or reaction medium in olefin oligomerization (Yanlong Gu et al, Catalysis Communications, 2004, 4, 597), etherification (Amanda C. Cole et al, J. Am. Chem. Soc., 2002, 124, 5962), esterification (Amanda C. Cole et al, J. Am. Chem. Soc., 2002, 124, 5962; Yanlong Gu et al, Journal of Molecular Catalysis A: Chemical, 2004, 212, 71; Jianzhou Gui et al, Catalysis Communications 2004, 5, 473), pinacol rearrangement (Amanda C. Cole, et al, J. Am. Chem. Soc., 2002, 124, 5962), Friedel-Crafts alkylation (Kun Qiao et al, Chemistry Letters, 2004, 33, 472) and Arene nitration (Kun Qiao, et al, Chemistry Letters, 2004, 33, 808). Recently, N-protonated Brønsted acidic ionic liquids based on methylimidazolium were also reported in a number of patents and literatures and could be used as novel liquid acidic catalysts in many acid-catalyzed reactions for replacing conventional inorganic acids, such as sulfuric acid, hydrofluoric acid etc. (Hai-Hong Wu, et al, Tetrahedron Letters, 2004, 45, 4963; Hua-Ping Zhu et al, Green Chemistry, 2003, 5, 38). These ionic liquids can be regarded as the third generation of ionic liquids.
Although dialkylimidazolium cation is quite suitable for constructing ionic liquids, a methylimidazole as a precursor thereof is expensive and available only in small scale because they have not been produced in large scale yet. Furthermore, the environmental compatibility of this kind of ionic liquids has been questioned because the toxicity of dialkylmethylimidazole cation is not so weak. So, the development of a new kind of cheaper and environmentally benign chemical feedstock with specific physicochemical properties as a precursor of cation of RTILs is of great scientific investigation value and practical significance.
Lactam and its derivates are amine derivatives that could be quaternized to act as a new kind of cation of RTILs. There is a carbonyl group in lactam molecules, and this may result in specific physicochemical properties and functions if they are incorporated as a cation into a RTIL. In comparison with conventional pyridines or alkylimidazoles, lactams and derivates thereof may intrinsically lower toxicity, and some lactams such as caprolactam have been produced currently in terms of megatons per year in chemical industry and is costed reasonably lower.
In 2002, it is reported that N-vinyl-N-alkylbutyrolactam ionic liquids through two-step reactions was prepared (D. Demberelnyamba et al, Chem. Commun., 2002, 1538). But these ionic liquids are generally neutral. So far, Brønsted acidic ionic liquids having N-protonated lactam cations have not been reported yet.